This invention relates to an alkali metal thermoelectric power generator which electrochemically converts thermal energy to electrical energy by means of alkali metal thermoelectric converters each using an alkali metal such as sodium or potassium and a solid electrolyte.
In an alkali metal thermoelectric converter (hereinafter referred to as "AMTEC"), an alkali metal is caused to move from a region on one side of a solid electrolyte to a region on the other side thereof by making use of the difference between the alkali metal density of each of the regions arranged respectively on each side of the solid electrolyte, thereby obtaining electrical energy. The basic construction of an AMTEC is disclosed in JP-B1-47-6660 and U.S. Pat. No. 3,535,163. In order to facilitate an understanding of the present invention, the principle of power generation on which the AMTEC is based will be schematically described below.
The AMTEC includes a power generator section, which is composed of a high temperature region (the first region) and a low temperature region (the second region), both being closed regions. The first and second regions are separated from each other by a solid electrolyte partition, which consists of beta (.beta.)-alumina (Na.sub.2 0.11Al.sub.2 O.sub.3), beta" (.beta.")-alumina or the like. The AMTEC is operated by heating the first region thereof. In the first region, sodium is heated to a temperature ranging from 900K to 1300K (the saturated vapor pressure thereof is in the range of 5.times.10.sup.3 to 3.times.10.sup.5 Pa) and exists in a vapor-liquid coexistent condition. The second region is a degassed vacuum chamber and includes a porous electrode which is in contact with the solid electrolyte partition and a condenser bordering on the vacuum chamber and facing the porous electrode. The condenser is cooled down to a temperature in the range of 400 to 800K. The sodium vapor pressure corresponding to the temperature of this condenser is in the range of 2.times.10.sup.-4 to 1.times.10.sup.2 Pa
The solid electrolyte (beta"-alumina in this case) exhibits ionic conductivity for the ions of an alkali metal (sodium in this case) but acts as an insulator for electrons. The high temperature sodium in the first region discharges electrons on one surface of the solid electrolyte, and the sodium ions are driven by the difference between the sodium density of the first and second regions, respectively, and pass through the solid electrolyte, reaching other surface of the solid electrolyte which faces the condenser. Meanwhile, the electrons, having moved from the liquid sodium in the first region via an external electric circuit, neutralize the sodium ions at the interface between the solid electrolyte and the porous electrode adjacent thereto. The neutralized sodium absorbs heat of vaporization while it is being diffused within the porous electrode and is evaporated in the vacuum chamber. The vapor phase sodium in the vacuum chamber condenses in the condenser at a low temperature. The liquid phase sodium generated through this condensation is returned to the first region by a pump. To summarize the above description, the difference between the sodium density of each of the regions provided respectively on each side of the solid electrolyte serves as the driving force for the sodium ions in the solid electrolyte, electrical power being generated through the movement of the ions. This power generation is effected as an endothermic reaction. The requisite heat for the power generation is supplied by heating the first region, and, to maintain the temperature difference between the condenser, which is in the second region, and the first region, while the condenser is cooled.
Generally speaking, in the AMTEC, as in other types of power generation systems, the higher the temperature on the high temperature side, the higher is the power generation efficiency that results, assuming that the temperature on the low temperature side is kept constant. Accordingly, it is necessary to set the temperature in the first region, and in particular, the temperature of the solid electrolyte, as uniformly high as the heat resistance of the components of the first region, i.e., the solid electrolyte, etc. permits. A method of effecting such a temperature setting is disclosed in "Radioisotope Powered Alkali Metal Thermoelectric Converter Design for Space System" by R. K. Sievers, et al., 1988 IECEC Proceeedings, Vol. 3, pp. 159-167 (1988). The above paper discloses an AMTEC structure in which heat is supplied by utilizing the principle of a heat pipe (hereinafter, this AMTEC will be referred to as "heat pipe type AMTEC or thermoelectric converter"). FIG. 5 shows the essential part of this structure. The AMTEC shown in FIG. 5 has a tubular body with one end open which includes an evaporator 311, and a solid electrolyte tube 302, which is also a tubular body with one end open. The inner space of the tubular body formed by connecting the respective open ends of these tubes with each other constitutes the first region serving as the high temperature region. This AMTEC also has a condenser shell 305, which is formed as a tubular body having a diameter sufficiently larger than that of the above-mentioned tubular body and which houses the solid electrolyte tube 302 with a sufficient clearance therebetween, and a porous electrode 303, which covers the outer periphery of the solid electrolyte tube 302. The space defined between the condenser shell 305 and the porous electrode 303 constitutes the second region. The inner peripheral surface of the above-mentioned tubular body which defines the first region is lined with a wick 312. The thermal energy from a heat source is added to the evaporator 311. The sodium evaporated by this thermal energy moves to the region of the solid electrolyte tube 302 and condenses on its inner surface, thereby imparting thermal energy to the solid electrolyte tube 302. A major part of the sodium that has condensed is returned to the evaporator 311 through the wick 312. The remaining sodium passes across the wall of the solid electrolyte tube as positive ions. The electrons inside the solid electrolyte tube 302 move via the wick 312 filled with sodium, the evaporator 311, etc. and are extracted through a cathode 313 provided in an electromagnetic pump 307. These electrons are conducted through an external circuit and an anode 314 to be returned to the porous electrode 303 provided on the outer peripheral surface of the solid electrolyte tube 302. The sodium ions passing through the solid electrolyte tube 302 are recombined with the electrons at the interface between the solid electrolyte tube 302 and the porous electrode 303. The sodium that is thereby neutralized absorbs heat of vaporization from the porous electrode 303 while diffusing within this electrode. This vapor phase sodium condenses on a wick 315 which is provided along the inner surface of the condenser shell 305. The condensed sodium is conducted via this wick 315 and a sodium return line 316a and is returned to the inlet of the electromagnetic pump 307. This electromagnetic pump 307 overcomes the pressure difference between the first and second regions and returns the sodium through a sodium return line 316b to the evaporator 311, thereby continuing the operation of the AMTEC. In this AMTEC, the first region acts as a heat pipe, so that the temperature gradient in the first region is small. The reference numeral 320 in the drawing indicates foil insulators for reducing the radiation heat loss from the AMTEC.
The inventors of the present invention examined and evaluated the AMTEC shown in FIG. 5 as follows:
The output density of the AMTEC is approximately 0.5 W/cm.sup.2 or so per unit area of the solid electrode and its output voltage is 0.7 V at most. Thus, to obtain an electrical power output of 1 kWe with a single AMTEC, it is necessary to extract a large electric current of approximately 1400A, so that, even when using copper, which is a good conductor, it is necessary to prepare a very thick conductor whose diameter is approximately 1 cm or more as well as a power converter for substantially augmenting voltage. Therefore, in actual practical use, a number of relatively small thermoelectric converters have to be connected in series to one another. However, in connecting a large number of heat-pipe-type thermoelectric converters in series to one another, the following technical problems remain to be solved:
(1) To attain certain improvements, such as reducing weight, simplifying the system configuration, or reducing cost, a plurality of thermoelectric converters have to be managed with a common pump. When using heat-pipe-type AMTECs, it is necessary to provide vapor space in the respective first regions thereof and to supply sodium at flow rates corresponding to the respective generation outputs of the thermoelectric converters. Otherwise, an excess or deficiency in the sodium amount in the first regions would occur, thereby hindering the power generation.
(2) To enhance the power generation efficiency of the entire system, it is necessary to diminish the temperature difference between the respective first regions of the thermoelectric converters. For this purpose, it is necessary to equalize the respective temperatures of the heat sources or effect heat exchange between the thermoelectric converters. Generally speaking, however, it is difficult to equalize the respective temperatures of the heat sources. This is particularly true of heat sources utilizing solar energy condensed by a lens, a reflector or the like, since such heat sources involve an incident energy difference per unit area between the respective heat collector sections of the thermoelectric converters. Further, no method has been previously reported of efficiently effecting heat exchange between the thermoelectric converters while retaining the electrical insulation between the respective first regions of the thermoelectric converters. To electrically connect thermoelectric converters in series to one another, the bypass electrical resistance between the thermoelectric converters must be greater by at least two digits than the internal resistance of the thermoelectric converters.